Feat/device policy

[laggui]

Checklist

• Confirmed that cargo run-checks command has been executed.
• Made sure the book is up to date with changes in this PR.

Related Issues/PRs

Fixes #3642

Changes

Step towards moving away from backend default element type generics (e.g., Cuda<f32, i32>).

This PR introduces a new API to set per-device default element type(s), e.g.

let device = B::Device::default();

// Update the device policy
set_default_float_dtype(&device, DType::F16);

// All float tensors created after this will use F16 by default
let tensor = Tensor::<B, 2>::zeros([2, 3], &device);

This is currently set as a public function, but once the multi backend lands it will be a method on a new general device type.

Testing

Describe how these changes have been tested.